One way to do this is if you can express one of the two variables in terms of the other variable.
So, for example, if you have a problem with variables x and y, you can write it just using x if you know the relationship between x and y.
Here is an example.
Let us say you have an equation that is originally
2x + y + 12
That has two variables. But if you know y is 3 more than x, you can express y as x + 3.
Then your equation could be
2x + x + 3 = 12.
Well, the first step is to establish a reference point and we'll write the other unknowns depending on this reference point.
For instance, in a box, there are 50 candies. The number of orange candies is 20 more than the number of mint candies. How many orange and mint candies are in the box?
We'll put the number of mint candies as the reference point: x .
total number of candies = 50.
The orange candies: x + 20
The total number of candies is:
mint candies + orange candies:
x + (x + 20) = 50
2x + 20 = 50
2(x+10) = 50
x+10 = 25
x = 15 mint candies
orange candies: 20 + 15 = 35 orange candies